Industrial applications are increasingly automated, with machines taking over the lion's share of the physical workload in industry. Most industrial machinery is driven by electric motor drives that push, pull, rotate, or otherwise manipulate industrial equipment under the supervision of skilled engineers who orchestrate the many complex and subtle movements necessary to bring a product through an assembly line. These drives can take many different forms and are produced in many different designs, but share many characteristics. Electric motor drives are also imperfect creations made from imperfect materials, and are susceptible to faults. While not all imperfections have been eliminated, those remaining tend to be subtle and difficult to detect using traditional methods.
In the field of electric motor drives, a motor is typically connected to a drive, which provides electrical power to the motor in a controlled manner. The motor-drive systems are commonly employed to provide speed control or torque control in an industrial application setting. For AC motors, electrical power is converted in the drive from an AC power supply (typically from a utility) into DC voltage. The DC voltage is then converted, using an inverter, into AC voltage (current) in which the frequency and amplitude can be controlled providing motor speed and torque controllability.
The existing techniques for ground fault detection are based on the measurement of overload current of two or more phase lines. Another method measures the common-mode current and compares the measured value with a threshold value. However, the normal capacitive leakage current value could be comparable to ground fault current caused by a high impedance ground fault. High impedance faults are short circuit faults with fault currents smaller than what a traditional over-current protective relay (or fuse) can detect. The currents caused by these faults are smaller in magnitude than the thresholds of traditional ground fault detection devices, so reliable detection of these high-impedance faults is challenging.